The present invention relates to joining pipes. This can be achieved in many ways, one of which is welding, which is used particularly in the case of large-diameter pipes, such as pipe-lines for gas, oil or other industrial products. The pipes may even be for district heating systems. In such pipes the demands for tightness and strength are generally extremely high and accurate welding methods are therefore necessary. Such pipes often have to be welded in unfavourable weather conditions and out at building sites. Due to the high demands on the finished welds, the use of welding becomes expensive and time-consuming. Welding also requires skilled labour. A normal welding joint cannot prevent running cracks which can occur in pipe-lines. These cracks have a tendency to run along a pipe-line for several kilometers, even passing welded joints.